1. Field of the Invention
The present invention relates an imaging device having a light modulation element for expanding a dynamic range of an imaging element.
2. Description of the Related Art
In imaging devices such as digital cameras, imaging is performed by performing photoelectric conversion on a subject image through an imaging element (a CCD sensor, a CMOS sensor, or the like), and image data is generated. However, the imaging element has a dynamic range (gray level reproducibility) lower than that of a conventional silver halide film, and is unable to sufficiently reproduce a gray level of a subject. For example, assuming that exposure is adjusted for a dark part of a subject, a luminance value of a captured image signal of a bright part of the subject becomes saturated, and thus a so-called white coloration occurs.
For this reason, the following technology was proposed: there is provided a plurality of segments, a light modulation element, of which a light transmittance is variable for each segment, is disposed on the front side of the imaging element, and imaging is performed in a state where the transmittances of the segments corresponding to the bright part of the subject are lowered and light is dimmed, thereby preventing the white coloration (refer to JP2003-348439A and JP2001-333329A). Thereby, the dynamic range of the imaging element is artificially expanded. In JP2003-348439A, as the light modulation element, an electro-chromic (EC) filter is used. In JP2001-333329A (corresponding to U.S. Pat. No. 7,245,325), as the light modulation element, a liquid crystal filter is used.
Assuming that a segment of the light modulation element is provided for each single pixel of the imaging element, light modulation control is performed with high accuracy. However, the number of pixels of the imaging element is large, and thus components such as a controller and a memory for controlling segments corresponding to the number of pixels are necessary. Hence, in JP2003-348439A and JP2001-333329A, a single segment can be allocated to a plurality of pixels so as to decrease the number of segments.
However, in a case where a single segment is allocated to a plurality of pixels, assuming that a transmittance is controlled for each segment, luminance values of a plurality of pixels corresponding to a single segment are increased or decreased together. Therefore, differences in level occur in the luminances of the pixels, between segments having different transmittances.
Accordingly, in the description of JP2003-348439A, the level difference is made unnoticeable by adjusting a gain of signal values of pixels near the boundaries of the segments having different transmittances. In the description of JP2001-333329A, more specifically, the level difference is made unnoticeable by performing low pass filter processing which is one type of digital filter processing.
However, a single segment includes not only a whiter region (high luminance region) but also a less white region (low luminance region). Thus, the low luminance region is dimmed together with the whiter region, and the luminance decreases. Assuming that the low luminance region stretches over the boundaries between segments having different transmittances, differences in level in the luminance occur. Level-difference correction processing described in JP2003-348439A and JP2001-333329A is processing of making only the boundaries between the segments unnoticeable. Therefore, it is not possible to eliminate the differences in level between segments themselves in the low luminance regions.